Manipulation Of Magnetic Moment Using The Spin Current From Magnetic And Non-Magnetic Materials
This thesis summarizes my studies on the effect of spin transfer torque on metallic ferromagnets. The spin current for generating the spin torque is either from ferromagnetic electrode through the spin filtering effect, or from nonmagnetic material through the spin Hall effect (SHE). In the experiment using the spin filtering effect, the current flows vertically through the nanoscale spin valve geometries. I will describe the fabrication process that I used to make the nanopillar structure and the strategy that I developed to reduce the critical current. In the experiment utilizing the SHE, the current flows within the film plane and the spin current is injected transversely from the non-magnetic (NM) film into the adjacent ferromagnetic (FM) layer. I will present five studies that I made to characterize the properties of the SHE and its influence on the magnetic moment. In the first study, I employed the spin torque ferromagnetic resonance (FMR) technique to determine the spin Hall angle. In this experiment, radio frequency current was applied onto the NM/FM bilayer and FMR was induced by the resultant oscillating spin current. By looking into the amplitude of the FMR signal, I was able to get the value of the spin Hall angle. In the second and third studies, I demonstrated that the SHE could be utilized to switch magnetic moment of both perpendicularly and in-plane magnetized FM films. For the perpendicular case, the spins injected into the FM film exert a torque that is perpendicular to the equilibrium position of the moment and it fights against the restoring anisotropy field and induces switching. For the in-plane case, the spins cause switching through the anti-damping mechanism. In the fourth study, I showed that the SHE could induce persistent magnetic oscillations. DC spin current reduces the magnetic damping to zero and the moment undergoes precession around the applied magnetic field. In the final study, I demonstrated that the SHE switching current can be modulated by the electric field applied across the FM/oxide barrier. The electric field modifies the anisotropy of the free FM layer and alters the critical current correspondingly.
Buhrman, Robert A
Van Dover, Robert B.; Ralph, Daniel C
Ph. D., Applied Physics
Doctor of Philosophy
dissertation or thesis